Wireline is a method of lowering specialised equipment into an oil or gas well, or raising specialised equipment from an oil or gas well. The principle of wireline is to attach a workstring or toolstring to the end of a reel of wire and by reeling out the wire the toolstring is lowered into the well. By either reeling in or reeling out the wire, the toolstring can be made to perform simple tasks downhole.
Wireline operations are frequently performed in live high pressure wells. The wireline used is mainly single strand high tensile wire (slick-line wireline), although multi-strand cable is also used to a lesser extent. An obvious problem however is that the wireline must be allowed to run free in the well and at the same time the well pressure must be contained. This is generally achieved by running the wire through a device known as a "Stuffing Box" for a single strand wireline, or through a device known as a "Grease Injection Head" for a multi-strand wireline.
Both methods involve setting up pressure control equipment at the surface which is connected directly onto the wellhead. However, equipment used to run a multi-strand wireline is more complex and expensive than the equipment used to run a single strand wireline. However, the advantage of running a multi-strand wireline is that it allows a greater pulling force to be achieved. Typically, this is about one and a half times as much force as can be pulled using a comparable single strand wireline. The expense of using a multi-strand wireline means that the weaker single strand wireline is most commonly used in wireline operations.
A further problem with conventional wireline operations is that force applied by the winch at the surface is greatly diminished at the toolstring, especially at great depth, due to wire stretch.
One of the most common operations performed by slick-line wireline is to set a series of plugs into the well-bore to hold back the natural flow of the well. This is done to enable flow control valves at the surface, that is the Xmas Tree, to be removed or repaired etc. safely. The plugs used are of a type which are designed to be located into a "landing nipple" which corresponds to each plug. The landing nipple consists of a "no-go" shoulder to land on and a recess for the plug to lock into.
The landing nipples are an integral part of the tubing string and are incorporated at various depths by the requirements of the Petroleum Engineer at the time of well completion. After a duration of time and depending on the sand content of the production fluid the landing nipple become "washed out". That is the "no-go" shoulder and recess are eroded away, when this occurs it is impossible to install a conventional plug. This condition is becoming a very common occurrence in oil wells in the North Sea.
A "washed-out" nipple system on a tubing string poses a very obvious problem to the operator, that being, how to install plugs to carry out the maintenance etc., as described above. At present there is only one answer to this problem, this being, to install electrically set bridge plugs. Bridge plugs can be installed anywhere in the tubing string by means of a toothed slip mechanism which allows these plugs to grip the internal diameter of the tubing. A compression sealing element seals against the internal diameter of the tubing thus forming the plug. A chemical charge is detonated by an electrical impulse sent through the cable and this detonation energises the setting mechanism downhole, to perform the setting process.
However, electric line methods incur high costs to the operator which means that setting bridge plugs is a very costly exercise. In addition, the force required to compress the sealing element increases as the sealing element is compressed but with a chemical charge which is detonated the maximum amount of compression force on the sealing element occurs at the initial detonation of the charge and decreases as the amount of compression force required to compress the sealing element increases. Hence, using a chemical charge is a relatively inefficient method of activating the bridge plug. There is also the danger of the charge being detonated inadvertently, for example by signals from radio or by electrical noise from other equipment on the rig. Hence, the handling and use of this type of equipment can be dangerous, especially in an offshore environment where there may be a high fire risk.